Organic dyes have attracted considerable attentions in the field of diode-laser optical storage. Recordable Compact Discs, so-called CD-R, as the first example of discs using this technology, are known from “Optical Data Storage 1989”, Technical Digest Series, Vol. 1, 45 (1989). They are writable at a wavelength of from 770 to 830 nm and readable at a reduced readout power. As recording media it is possible to use, for example, thin layers of cyanine dyes (JP-58/125246), phthalocyanines (EP-A-676 751, EP-A-712 904), azo dyes (U.S. Pat. No. 5,441,844), double salts (U.S. Pat. No. 4,626,496), dithioethene metal complexes (JP-A-63/288785, JP-A-63/288786), azo metal complexes (U.S. Pat. No. 5,272,047, U.S. Pat. No. 5,294,471, EP-A-649 133, EP-A-649 880) or mixtures thereof (EP-A-649 884). Such an organic colorant type optical recording medium, by being decomposed under laser irradiation changes its optical characteristics, induces a decrease in the layer thickness, as well as a subsequent deformation of the substrate.
By using more advanced lasers, emitting in the range of from 600 to 700 nm it was possible to achieve a 6 to 8 fold improvement in recording density, in that the track spacing (distance between two turns of the information track) and the size of the pits as well as the redundancy each were reduced to approximately half the value in comparison with conventional CD-R's.
This new disc format, so-called recordable digital versatile discs (DVD-R), may contain, as recording layer, numerous dyes also based on phthalocyanine, hemicyanine, cyanine and metallized azo structures. These dyes are suitable in the respective field with the laser wavelength criteria. Other general requirements for a good dye media are strong absorption, high reflectance, high recording sensitivity, low thermal conductivity as well as light and thermal stabilities, durability for storage and non-toxicity.
Due to a large increase of the original recording speed, most of the known recording dye layers do not possess the required properties to a satisfactory extent.
Metal chelate compounds comprising azo ligands and metal, as proposed for example in JP-A-3-268994, are currently still the closest to high-speed requirements.
It has therefore been an objective of the invention to provide new dyes, suitable for high-density and high speed recording materials in a laser wavelength range of from 500 to 700 nm.
It further has been an objective of the invention that the new dyes are easy to synthesize with high yields and high purities at low costs. It has been found that dyes according to the invention meet the above objectives.